ABSTRACT
The Xiaoqinling metamorphic core complex (XMCC) is located at the southern margin of the North China Craton. Along the boundary of the XMCC is a detachment fault system: mylonitic rocks, chloritized breccias, microbreccias and fault surface. Many retrograded shear zones, normal faults and contraction faults developed within the XMCC. There are two groups of linear features in the XMCC according to their occurrences: one represents the earlier ESE-WNW extension and the other shows the normal faulting toward the outsides of the XMCC. Fabrics, strain and kinematic vorticity in the related mylonitic zone present the characteristics of a detachment fault. The strain increases from ESE to WNW and geometry of quartz c-axis fabrics changes progressively from crossed girdles on the ESE boundary to single girdle on WNW. In the same direction, the kinematic vorticity number increases, i.e., the ratio of pure shear rate to that of simple shear decreases. The ESE-WNW extension and the detachment began at ~75.9 Ma, as a result of crustal thickening caused by the crustal shortening during Yanshanian orogeny. The following uplift due to denudation resulted in the emergence of the XMCC. The retrograded shear zones and normal faults within the XMCC were formed by the collapse of the XMCC during late- and post-orogenic event.
